The present invention relates to a fail-safe control for an electronically controlled automatic transmission to be mounted on a vehicle, and, more particularly, to a fail-safe control against a failure of an engine water temperature sensor for an electronically controlled automatic transmission.
In the conventional electronically controlled automatic transmissions, the gear ratio of a T/M (transmission) and whether the lock-up clutch is turned on or off are determined in response to signals representing the degree of opening of a throttle and car speed so that an actuator such as a solenoid mounted on the T/M is controlled.
Furthermore, in order to improve driveability and engine protection, operation at the maximum gear ratio and the operation of a lock-up clutch are prohibited if the engine water temperature is determined to be too low in accordance with a signal representing the engine water temperature.
However, in the conventional method, if a engine water temperature sensor fails, the maximum gear ratio and the lock-up clutch cannot be operated although the engine temperature has been brought to a sufficiently high level, with the result that the fuel consumption becomes worse and noises are generated.